The presence of microorganisms in clinical specimens is conventionally determined by culturing the specimens in the presence of nutrients and detecting microbial activity through changes in the specimen or in the atmosphere over the specimen after a period of time. For example, in U.S. Pat. No. 4,182,656 to Ahnell et al. the sample is placed in a container with a culture medium comprising a carbon 13 labelled fermentable substrate. After sealing the container and subjecting the specimen to conditions conducive to biological activity, the ratio of carbon 13 to carbon 12 in the gaseous atmosphere over the specimen is determined and compared with the initial ratio. In U.S. Pat. No. 4,152,213, a method is claimed by which the presence of oxygen consuming bacteria in a specimen is determined in a sealed container by detecting a reduction in the amount of oxygen in the atmosphere over the specimen through monitoring the pressure of the gas in the container. U.S. Pat. No. 4,073,691 provides a method for determining the presence of biologically active agents, including bacteria, in a sealed container containing a culture medium by measuring changes in the character of the gaseous atmosphere over the specimen after a period of time. A method for non-invasive detection of CO.sub.2 changes in the gaseous atmosphere is taught by Suppman et al., as disclosed in EPO application 83108468.6, published Apr. 4, 1984. The methods and apparatus described in these and other publications all require either a radiometric method or the invasion of the sealed container to measure changes in the gaseous atmosphere after culturing or require special materials that permit infra-red light to pass.
Other known methods for measuring microbial presence in specimens, particularly blood cultures, include measuring minute changes in temperature, pH, turbidity, color, bioluminescence, and impedance. Generally, these methods determine microbial presence or growth by detecting bacterial metabolic byproducts. Microbial presence may also be assessed by subculturing and/or staining. Of these methods, only impedance, radiometry and infra-red spectrometry provide the possibility of automated processing of clinical specimens. And except for impedance and infra-red measurements, these procedures also require entering the container in order to make a measurement on the liquid specimen or the gaseous atmosphere over the specimen. In addition to the likelihood of contamination and creating the likelihood of altering the constituency of the atmosphere over the specimen each time a determination is made, these methods do not permit taking measurements continuously or repeatedly over short time intervals for an extended period of time. This is a significant disadvantage as the growth rate of organisms differs depending on the organism and the number of organisms in the original sample, such that it cannot be predicted when detectable changes in the atmosphere or fluid sample will be presented. In a related problem, when organism growth is determined by pH changes in the liquid sample, various metabolic products will affect the pH of the sample differently. For example, the production of ammonia will raise the pH while the production of CO.sub.2 will lower it. Different growth rates of different organisms could result in a pH increase at one time and a decrease at another time, which would not be detected if the pH is measured at widely spaced intervals. Another source of error when detecting changes by pH measurement in whole blood samples, particularly when an indicator dye is the means for pH determination, is the likelihood that the dye appearance can be affected or obscured by the presence of blood cells. Colorimetric indicators can only be effectively used if errors induced by the nature of the specimen can be prevented from influencing the appearance of the dye.
When the biologically active agent is an aerobic organism, a system must be provided for ensuring sufficient oxygen within the vessel so that biological activity can take place. One way of providing oxygen to the vessel is by adding oxygen to the atmosphere within the vessel containing the culture medium, at the time of manufacture of the vessel. Then, when a specimen is added to the vessel by the user of the vessel, oxygen will already be present within the vessel. Alternatively, a gas permeable membrane can be provided, such as within the cap of the vessel.
In order to overcome the problems associated with invasive measurement methods, measurement systems have been developed which utilize a sensor disposed inside the vessel. The sensor undergoes a change due to changes in amounts of a particular metabolic product or food source of the microorganisms. The sensor can be constructed so as to respond to changes within the vessel, thereby changing, for example, in color or fluorescence intensity. In the conventional fluorometric or colorometric measurement systems, a sensor which changes in color or fluorescence intensity is disposed on the inside of the culture bottle along the flat bottom surface of the bottle. A light source such as a light emitting diode can be provided proximate to the flat bottomed surface of each bottle, along with a detector for detecting changes in color or fluorescence. However, when an individual light source and detector are provided for each culture bottle, a degree of non uniformity is introduced and can result in errors of measurement.
In order to address this concern, rotatable culture systems have been proposed whereby a plurality of culture bottles are rotated past the same light source/sensor. For example, in U.S. Pat. No. 4,293,643 to Ohtake et al., a rotary culturing and measuring system is disclosed where L-shaped culture tubes are disposed radially around a rotatable drum at equal intervals. The drum (turntable) is disposed at an angle to horizontal, and a central shaft is driven to continuously or intermittently index the drum. Due to the movement of the drum, the substance being cultured is shaken. At a particular position along the wheel, the growth of the substance being cultured in the L-shaped culture tubes, is measured. A light source and a photodetector are disposed on opposite sides when one part of the L-shaped culture tube passes. While the drum turns, the turning motion allows for the use of a single light source and sensor to obtain the measured concentration values signal at every fixed time (or the degree of growth of a microorganism can be observed over a particular time interval).
In published European patent application EP 609986, a plurality of vials for culturing microorganisms are placed in a drum and rotated about an axis. Agitation results from placing the axis of rotation perpendicular to the force of gravity. A single light source and detector are provided (for each detection method) such that a plurality of vials may utilize a common light source and sensor.